Field of the Disclosure
The present disclosure relates to an apparatus for delay angle compensation of flying start function. More particularly, the apparatus according to the present disclosure enables a medium voltage inverter to be restarted using a flying start function especially when electric power of the medium voltage inverter is restored after a trip or an instantaneous blackout occurs.
Discussion of the Related Art
There are various types of medium-voltage inverters used in industrial sites. For example, a multi-level medium-voltage inverter is an inverter having an input power above 600V as an effective value of a voltage between input lines. An output phase voltage of the multi-level medium-voltage inverter has multi-levels. Medium-voltage inverters are used for high-capacity motors having capacity of hundreds of kW˜tens of MW. The medium-voltage inverters are used in various fields such as fans, pumps, compressors, tractions, hoists, and conveyors.
Among the multi-level medium-voltage inverters, a cascaded h-bridge inverter is mainly used as the medium-voltage inverter, because the cascaded h-bridge inverter has a module structure to facilitate expansions.
In addition, a unit power cell may be formed variously, such as an h-bridge inverter, a single-phase NPC (Neutral Point Clamped) inverter, a single-phase T-type NPC inverter, etc.
FIG. 1 is an exemplary view illustrating a conventional medium-voltage inverter system (10), which includes a multi-winding phase-shifting transformer (11), a plurality of power cells (12), and a motor (13).
The multi-winding phase-shifting transformer (11) provides an electrical insulation between an electrical power input terminal and a medium-voltage inverter, reduces input terminal harmonics, and provides a three phase input power supply appropriate to each of the power cells (12). The power cells (12) output phase voltages of the motor (13) by being supplied with a power supply from the multi-winding phase-shifting transformer (11). The motor (13) is a high-voltage three-phase motor. An induction motor or a synchronous motor may be used as the motor (13).
FIG. 2 is an exemplary view illustrating a structure of a power cell (12), which includes a three-phase diode rectifier (12-1), a DC (Direct Current)-link capacitor (12-2), and a single phase full bridge inverter (12-3).
The three-phase diode rectifier (12-1) receives an input of three-phase AC (Alternating Current) voltage, rectifies the three-phase AC voltage with a diode, and output as a DC (Direct Current) voltage. The DC-link capacitor (12-2) maintains the DC voltage outputted from the diode rectifier (12-1), and compensates an instantaneous difference in electric power. The single phase full bridge inverter (12-3) synthesizes output voltages from a DC terminal.
Meanwhile, the motor (13) driven by such medium-voltage inverter generally has a high inertia. Thus, a rotor speed of such motor (13) is not reduced greatly even when the inverter cannot operate normally due to causes such as an abnormality in power supply, a blackout, etc. Therefore, when electric power of the medium voltage inverter is restored from abnormality, it takes long time to restart the inverter after waiting until the rotor speed comes to a zero speed.
To avoid such problem described in the above, it is advantageous to restart the medium-voltage inverter by estimating a rotor speed of the motor (13) when the electric power is restored to a normal state. This function is referred to as “Flying Start”.
FIG. 3 is an exemplary view illustrating a conventional method to set an initial angle for flying start. The method includes detecting a three-phase voltage from an output terminal of a medium-voltage inverter (input terminal of a motor), calculating a phase angle (32) through a PLL (31: Phase Locked Loop), and using the calculated phase angle (32) as an initial angle for the flying start.
However, in a conventional method, there occurs a problem that a command voltage phase angle of the inverter differs from an actual output voltage phase angle due to a delay caused by a sampling and/or a filtering. The motor may be damaged by an inrush current generated greatly, when the flying start function is started in a state when the command voltage phase angle of the inverter and the actual output voltage phase angle are different from each other.